In recent years, the zoom lens has become a popular feature of a lens shutter type camera and a digital still camera particularly a zoom lens with a high zoom ratio of over 3.times.. The zoom lens with a high zoom ratio of this type basically uses zoom lenses having more than three lens groups which are movable during zooming.
A large number of zoom lenses of conventional technology having a field angle of about 60.degree. have been proposed. Unlike a single reflex camera in which a lens system is switchable, the lens system and camera body are formed integrally which contributes to a reduction in the size of the camera body. As a result, a variety of compact zoom lens inventions have been proposed.
In the digital still camera, a solid state image sensor such as a charge coupled device (CCD) is used for the lens system. A CCD is made up of photo sensors, which have a capability of photoelectric conversion. A potential difference corresponding to the light intensity is measured in each of the photo sensors. However, each of the detector devices is microscopic, therefore, the level of light intensity which arrives at each of the photo sensors is so small that it is susceptible to noise interference. Therefore, a micro lens array is arranged immediately before each of the photo sensors to increase the light beams which reach each of the photo sensors. Due to the presence of the micro lens array, if the exit pupil of the lens system is located near a CCD, the light beams which should reach peripheries of the image plane do not reach a photo sensor, thus causing a lack of light intensity. For this reason, there is a limitation in the lens system which uses a solid state image sensor such as a CCD that the distance between the exit pupil and the image plane be sufficiently apart as in the case, for example, for a digital still camera lens system.
FIG. 36 shows a refractive power layout of the image side telocentric optical system, its exit pupil is at the infinitely far range. In FIG. 36, lens system L comprises lens group L1, which is arranged more toward the object side over aperture stop S, and lens group L2, which is arranged more toward the image side over aperture stop S. A CCD is located at position I, which is the image plane of the lens system L. In FIG. 36, as principal ray R indicates, an image of aperture stop S (that is the exit pupil) made by lens group L2 is at the infinitely far range. Therefore, in order to reduce the distance between aperture stop S and the image plane I (where a CCD is), the convergence capability of lens group L2 needs to be increased and at the same time, the thickness of lens group L2 needs to be decreased in the optical axial direction. As such, the telocentric optical system in which the exit pupil is positioned completely at the infinitely far range is not suited to reducing the overall lens length.